Channel Sounding for Frequency Division Duplex System

ABSTRACT

System and method embodiments are provided for channel sounding in a frequency division duplex (FDD) system. The embodiments enable a transmission point (TP) to determine channel information about a downlink channel from an uplink channel sounding signal received on the downlink channel frequency band during a time window reserved for uplink channel sounding on the downlink channel frequency band. In an embodiment, a method in a controller includes determining with the controller a schedule for an uplink sounding window in a downlink frequency band, wherein the uplink sounding window comprises a transmission window in at least a partial downlink frequency band that is reserved for uplink channel sounding, instructing a TP to signal the schedule to at least one wireless device in a coverage area of the TP, receiving a channel sounding signal in the downlink frequency band, and obtaining downlink channel state information from the channel sounding signal.

TECHNICAL FIELD

The present invention relates to a system and method for wirelesscommunications, and, in particular embodiments, to a system and methodfor channel sounding for a frequency division duplex system.

BACKGROUND

New technologies such as coordinated multi-point (CoMP), interferencealignment (IA), dirty paper coding (DPC), massive multiple-inputmultiple-output (MIMO), etc. may be some of the keys to capacityenhancement for wireless systems. However, all of the benefits providedby these technologies may not be realized due to the requirements forprecise channel knowledge. For a frequency division duplex (FDD) system,various channel feedback schemes have been proposed. However, theoverhead, accuracy, and feedback delay are still major roadblocks.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a method in a controller for downlinkchannel sounding in a frequency division duplex (FDD) wireless systemincludes determining with the controller a schedule for an uplinksounding window in a downlink frequency band, wherein the uplinksounding window comprises a time window in a downlink frequency bandthat is reserved for uplink channel sounding; instructing a transmissionpoint (TP) to signal the schedule to at least one wireless device in acoverage area of the TP; receiving a channel sounding signal in thedownlink channel frequency band from the at least one wireless device;and obtaining downlink channel state information at the TP from thechannel sounding signal.

In accordance with another embodiment, a network component configuredfor downlink channel sounding in a frequency division duplex (FDD)wireless system includes a processor and a computer readable storagemedium storing programming for execution by the processor, theprogramming including instructions to: determine a schedule for anuplink sounding window in a downlink frequency band, wherein the uplinksounding window comprises a time window in a downlink frequency bandthat is reserved for uplink channel sounding; instruct a transmissionpoint (TP) to signal the schedule to at least one wireless device in acoverage area of the TP; receive a channel sounding signal in thedownlink frequency band from the at least one wireless device; andobtain downlink channel state information at the TP from the channelsounding signal.

In accordance with another embodiment, a method in a network componentfor determining downlink channel state information in a frequencydivision duplex wireless system includes determining with the networkcomponent a schedule for transmissions, wherein the schedule comprises adownlink (DL) transmission period for DL data transmission in a DLfrequency carrier to wireless devices in a coverage area and an uplink(UL) sounding period for UL channel sounding in the DL frequencycarrier; signaling the schedule to the wireless devices; transmitting DLdata to at least one of the wireless devices during the DL transmissionperiod; receiving UL sounding signals in the DL frequency carrier fromat least one of the wireless devices during the UL sounding period; anddetermining DL channel state information according to the UL soundingsignals.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a network for communicating data;

FIG. 2 is a graph illustrating an embodiment schedule for UL soundingand DL transmission;

FIG. 3 is a diagram illustrating an embodiment communication system thatprovides for co-existence of different sounding window configurationsfor different APs;

FIG. 4 is a graph of embodiment sounding window schedules for the APsdepicted in FIG. 3;

FIG. 5 is a diagram of an embodiment communication system forinterference management while performing UL sounding in the DL frequencycarrier;

FIG. 6 is a graph showing UL sounding window schedules for neighboringAPs for interference management;

FIG. 7 is a flowchart of an embodiment method for determining channelstate information about a DL channel;

FIG. 8 is a flowchart of an embodiment method for managing interferencefrom neighbor UEs transmitting UL sounding signals in a DL frequencycarrier; and

FIG. 9 is a processing system that can be used to implement variousembodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

In FDD, downlink (DL) channel sounding is done through per antennaelement based DL pilot measurement and report. Pilot overhead increaseslinearly with the number of transmit antennas N_(T). Feedback overheadincreases at a rate of N_(T)N_(R) where N_(R) is the number of receiveantennas. For a large-scale (LS) multiple input, multiple output (MIMO)based system, both overheads become prohibitive for practicalimplementation. In addition, the transmitter cannot obtain full channelstate information if the DL channel sounding is done with a low overheadchannel state information (CSI) feedback scheme.

Uplink (UL) pilot based DL channel sounding is beneficial. It providesfull CSI at the transmitter (CSIT) to the transmitter and allows betterprecoding, especially for multi-user (MU) MIMO. Furthermore, the pilotoverhead does not increase with N_(T). Therefore, in a LS-MIMO basedsystem, since N_(T)>>N_(R), the pilot overhead for channel estimation ismuch smaller. However, in a FDD system, the DL and the UL signals aretransmitted in different frequency bands. Thus, channel reciprocity nolonger holds. Because of this and due to much narrower beam-widths inLS-MIMO, a more reliable way for obtaining accurate CSIT is needed.

Disclosed herein is a system and method for FDD DL channel sounding toprovide complete CSIT to a transmission point (TP), an access point(AP), or a base transceiver station (BTS). In an embodiment, a ULsounding window is provided and reserved in the DL carrier. The locationand the duration of the UL sounding window is dynamically orsemi-statically configurable by the BTS, the TP, a central server, or acontroller, and may depend, for example, on the number of DL active userequipment (UEs) (i.e., wireless devices) and the mobility of the UEswithin the coverage area of the BTS. The controller may be the centralserver or may be the TP. In an embodiment, the controller is a master TPthat also controls other TPs. In an embodiment, the location and theduration of the UL sounding window also depends on other factors such asthe DL M-MIMO transmission mode to the UEs. The UE sends soundingsignals during this period in the DL frequency band thereby providingthe network with DL channel information by taking advantage of channelreciprocity.

In an embodiment, the uplink sounding window includes a transmissionwindow in at least a partial DL frequency band that is reserved for ULchannel sounding. Thus, only a portion of the DL frequency band isreserved for UL channel sounding.

In an embodiment, different sounding window configurations co-exist inneighboring coverage areas belonging to neighboring BTSs. Furthermore,since the number of UEs and the mobility of UEs within a coverage areafor a given BTS may change dynamically, the sounding windowconfigurations may change dynamically in each BTS. Neighboring BTSs mayexchange information through a backhaul network in order to coordinatetheir UL sounding window schedules to mitigate any effects frominterference caused by other UEs or BTSs transmitting at the same timeon the same DL channel resources.

In another embodiment, a central server or controller provides each BTSwith a UL sounding window schedule. The central server or controllerreceives information from each BTS regarding the number of UEs in theBTS's coverage area and the mobility of the UEs and uses the informationto determine a UL sounding window for each BTS that mitigatesinterference between the UEs and the BTSs. The schedule may bequasi-static or may change dynamically as the conditions warrant.

When there is an overlap of DL transmission period and UL soundingwindows between adjacent or neighboring UEs, interference may occur orbe experienced by a UE receiving a DL signal while a neighbor UE issending a sounding signal. In various embodiments, interferencemanagement schemes are utilized to mitigate or reduce the effect of theinterference on the UE receiving the DL signal. In an embodiment, the APtransmitting the DL signal to the UE utilizes beamforming to enhance theDL signal strength such that the DL signal strength is much greater thanthe strength of the sounding signal. In another embodiment, the UEincludes multiple receiver antennas that may be used to reject theinterference from the sounding signal. Techniques for using multiplereceiver antennas to reject interference are well known to those ofordinary skill in the art. In yet another embodiment, the scheduling ofUL sounding windows for UEs that are near each other, but that arecommunicating with different APs are determined such that DLtransmission (i.e., a DL signal) to a UE is avoided during a time orfrequency band when/where a nearby or neighboring UE is transmitting asounding signal.

In an embodiment, sounding window configuration information is sent tothe UEs by the TP through broadcast signaling or multicast signaling.The sounding window configuration information may be sentsemi-statically or dynamically.

In an embodiment, in order to control the DL-UL interference cause bythe asynchronous sounding window settings, the sounding windowconfiguration information is shared by neighboring transmitters. As usedherein, in an embodiment, the term neighboring AP or neighbor AP refersto any AP that is adjacent to or near enough to another AP such thattransmissions targeting to a first AP may cause interference in UEsassociated with the other AP or with the other AP, or another AP that isassociated with a UE that is near enough to a UE associated with thefirst AP such that transmissions by one UE associated with one AP maycause interference in the UE associated with the other AP. An AP mayhave more than one neighbor AP. As used herein, a UE is considered aneighbor of another UE if the two UEs are close enough that transmissionin one may cause interference in the other. In an embodiment, thesounding window information exchange between APs is performed in adistributed manner where information may be exchanged betweentransmitters through a backhaul network. In another embodiment, thesounding window information exchange between APs is performed in acentralized manner in which each transmitter (e.g., AP) reports thesounding window configuration information to a central server and thecentral server sends the neighbor's configuration information to theother transmitters.

In an embodiment, because of channel reciprocity, the BTS uses the ULpilot signal transmitted by the UEs in the DL carrier to obtain completeCSIT information in the DL channel.

FIG. 1 illustrates a network 100 for communicating data. The network 100comprises a plurality of TPs 110 each having a coverage area 112, aplurality of user equipment (UEs) 120, a backhaul network 130, and acentral server 140. In some embodiment, the coverage areas 112 mayoverlap. As used herein, the term TP may also be referred to as an AP aBTS and the three terms may be used interchangeably throughout thisdisclosure. The TP 110 may comprise any component capable of providingwireless access by, inter alia, establishing uplink (dashed line) and/ordownlink (dotted line) connections with the UEs 120, such as a BTS, anenhanced base station (eNB), a femtocell, and other wirelessly enableddevices. The UEs 120 may comprise any component capable of establishinga wireless connection with the TP 110. Examples of UEs include smartphones, laptop computers, and tablet computers. The backhaul network 130may be any component or collection of components that allow data to beexchanged between the TP 110 and a remote end (not shown) and with thecentral server 140. In some embodiments, the network 100 may comprisevarious other wireless devices, such as relays, femtocells, etc.

In an embodiment, the network 100 is an FDD network and the TPs 110schedule a UL sounding window in the DL carrier for performing DLchannel sounding. Each TP 110 schedules the UL sounding window for theUEs 120 in its coverage area 112. If the TPs 110 are neighbors andutilize the same frequency bands, then the TPs 110 may communicate witheach other to jointly schedule UL sounding periods, UL soundingopportunities and/or DL transmission opportunities (or channel resourcesin the time domain and frequency domain) for UEs close to the boundaryof the coverage area such that neither TP 110 nor their UEs 120experience negative consequences from interference caused by multiplesimultaneous transmissions on the same frequency band. However,typically, the interference is suffered by the UE which is receiving aDL signal due to the co-transmission of a sounding signal from a nearbyUE while the effects on the TP 110 are minimal. The UL sounding windowschedule is dynamically adjustable by each TP 110 to substantiallyoptimize the use of resources. In an embodiment, the TP 110 dynamicallyadjusts the UL sounding period based on the number of UEs 120 in itscoverage area 112 and/or on the mobility of the UEs in its coverage area112 as well as the DL transmission mode and/or the DL traffic type ofthe UE 120.

In other embodiments, the central server 140 creates a schedule for ULsounding windows in the DL carrier for each TP 110. The central server140 dynamically adjusts the UL sounding window schedule for each TP 110based on changing conditions, such as, for example, the number of UEs120 in the coverage area 112 and the mobility of the UEs 120 in thecoverage area 112.

In an embodiment, the TP 110 employs beamforming to help the UEs 120distinguish the signals from the TP 110 from interference generated by,for example, a UE 120 performing UL sounding in a DL frequency carrierfor a neighbor TP 110.

In an embodiment, orthogonal sequences are allocated to cell edge UEs120 if the cell edge UEs 120 sound at the same time. As furtherexplanation, assume two cells: cell A and cell B. The UEs performingsounding in cell A using the same time-frequency resources will need touse orthogonal sequences. Further assume that UE-ab is a cell edge UE ofcell B, which is close to cell A. Then UE-ab should use a sequence thatis orthogonal to both the sequences used in both cell A and cell B. Thesequences used by cell center UEs in cell B, however, do not need to beorthogonal with those used by cell center UEs in cell A.

FIG. 2 is a graph illustrating an embodiment schedule 200 for ULsounding and DL transmission. As shown, the schedule 200 includes aplurality of DL transmission periods (DL-slots) 202 separated by a ULsounding period (UL-slot) 204. In an embodiment, the UL-slots 204 aretypically of a much shorter duration than the DL-slots 202.

FIG. 3 is a schematic diagram illustrating an embodiment communicationsystem 300 that provides for co-existence of different sounding windowconfigurations for different TPs. The system 300 includes multiple TPs302, 304 and each TP 302, 304 has a corresponding coverage area 306,308. The system 300 also includes a plurality of UEs 310, 312, 314, 316,318, 320, 322, 324, 326, and 328. UEs 310, 312, 314, 316, 318, 320, 322communicate with TP 302 and UEs 324, 326, 328 communicate with TP 304.Each TP 302, 304 may include more than one transmit node (i.e., virtualtransmit nodes). Each TP 302, 304 determines its own UL sounding periodschedule according to the number of UEs 310, 312, 314, 316, 318, 320,322, 324, 326, 328 within its corresponding coverage area 306, 308 aswell as other factors, such as mobility of the UEs and the amount ofdata to be transmitted to the UEs 310, 312, 314, 316, 318, 320, 322,324, 326, 328 within its corresponding coverage area 306, 308. Theschedules for each TP 302, 304 do not have to be the same and, in manyembodiments, may be different.

FIG. 4 shows the embodiment sounding window schedules 400, 450 for theTPs 302, 304 depicted in FIG. 3. Schedule 400 corresponds to TP 302 andschedule 450 corresponds to TP 304. Schedule 400 includes a plurality ofDL transmission periods 402 separated by UL sounding periods 404.Schedule 450 also includes a plurality of DL transmission periods 452separated by UL sounding periods 454. As shown, the length of timeallocated to the UL sounding periods 404 in schedule 400 is longer thanthe length of time allocated to the UL sounding periods 454 in schedule450. It should also be noted that the length of time devoted to a DLtransmission period 452 may vary over time such that different DLtransmission periods 452 have different time lengths, as shown.Additionally, the time length (i.e., time duration) for the UL soundingperiods 454 may vary over time as shown. Thus, the TP 302, 304 mayadjust or change the time periods devoted to the DL transmission periodand the UL sounding period depending on the conditions in the network,the number of UEs in the coverage area, the amount of data to betransmitted, as well as other factors. Therefore, two consecutive ULsounding windows (separated by a DL transmission period) may bedifferent.

FIG. 5 depicts an embodiment communication system 500 for interferencemanagement while performing UL sounding in the DL frequency carrier.System 500 includes multiple TPs 502, 504, each with a correspondingcoverage area 506, 508. The system 500 also includes a plurality of UEs510, 512, 514, 516, 518, 520, 522, 524, 526, and 528. UEs 510, 512, 514,516, 518, 520, 522 communicate with TP 502 and UEs 524, 526, 528communicate with TP 504. When there is an overlap of DL transmissionperiod and the UL sounding window between adjacent transmitters,interference may occur when the UE receiving DL signals and the UEsending sounding signals are close to each other. For example, as shown,when UE 524 transmits UL sounding signals in the DL frequency carrier toTP 504 at the same time that the TP 502 transmits DL signals in the DLfrequency carrier to UEs 520, 522, UE 522 may experience interferencecaused by the transmission of UE 524. For interference management, eachTP 502, 504 can DL beamform (BF) in order to enhance the DL signalstrength. In an embodiment, one or more of the UEs 510, 512, 514, 516,518, 520, 522, 524, 526, 528 may utilize multiple receiver antennas inorder to reject the interference. In another embodiment, in order tomanage the interference, the TPs 502, 504 coordinate their scheduling toavoid scheduling DL transmission to a UE 522 that is close to a UE 524that is transmitting a sounding signal. In another embodiment, in orderto manage the interference, the TPs 502, 504 coordinately schedule theUL sounding transmission of the UE 524 which is close to the UE 522 thatis receiving a DL signal over the same channel resources.

FIG. 6 is a graph showing UL sounding window schedules 602, 604 forneighboring TPs for interference management. Schedule 602 corresponds toa schedule for a TP (e.g., TP 504 in FIG. 5) without taking neighboringDL transmission schedules into account. Schedule 602 includes a DLtransmission period 604 and an UL sounding period 606. Schedule 612 is aschedule for the TP (e.g., TP 504 in FIG. 5) after taking into accountneighboring DL transmission from a neighbor TP (e.g., TP 502 in FIG. 5).Schedule 612 includes a DL transmission period 614 and an UL soundingperiod 616 and a delay period 618 during which the UEs (e.g., UE 524)will not transmit UL sounding signals so as to not cause interference ina neighboring UE (e.g., UE 522) receiving DL signals from its TP (e.g.,TP 502). The delay period 618 may correspond to a time when theneighboring UE (e.g., UE 522) receives DL signals from its TP (e.g., TP502). In an embodiment, other UEs (e.g., UE 526 and UE 528) in the TPscoverage area (e.g., coverage area 508) that are not near the neighborUE (e.g., UE 522) may transmit UL sounding signals during the delayperiod 618 since they are not close enough to the neighbor UE to causeinterference. In an embodiment, the UE 522 and the UE 524 are scheduledin different frequency bands to avoid causing interference for eachother.

FIG. 7 is a flowchart of an embodiment method 700 for determiningchannel state information about a DL channel. The method 700 begins atblock 702 where a TP determines the number and mobility of wireless DLactive devices in its coverage area. At block 704, the TP determines aschedule that includes a UL sounding period in the DL frequency carrieraccording to the number and mobility of wireless DL active devices inthe coverage area, the amount of DL data to transmit, as well as otherfactors. At block 706, the TP signals the schedule, including the ULsounding period, to the wireless devices by broadcasting, multicasting,or unicasting. At block 708, the TP receives the UL sounding signals (ortransmissions) in the DL frequency carrier from the wireless devices. Atblock 710, the TP determines DL channel state information according tothe received UL sounding signals received from the wireless devices,after which, the method 700 ends.

FIG. 8 is a flowchart of an embodiment method 800 for managinginterference from neighbor UEs transmitting UL sounding signals in a DLfrequency carrier. The method 800 begins at block 802 where the TPdetermines or obtains information about a neighbor TP's datatransmission schedule. The neighbor TPs may communicate with each otherto jointly schedule the UL sounding periods. At block 804, the TPdetermines active wireless devices in its coverage area that are nearthe coverage are of the neighbor TP. At block 806, the TP determines theschedule for the UL sounding window (in the time domain or in thefrequency domain) in the DL frequency carrier for the wireless devicesthat are near the coverage area of the neighbor TP such that thewireless devices are not scheduled to transmit UL sounding transmissionswhen the neighbor TP is scheduled to transmit DL data to nearby wirelessdevices. At block 808, the TP signals (e.g., broadcasts or multicasts)the schedule to the wireless devices in its coverage area, after which,the method 800 ends.

FIG. 9 is a block diagram of a processing system 900 that may be usedfor implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system 900 may comprise a processing unit901 equipped with one or more input/output devices, such as a speaker,microphone, mouse, touchscreen, keypad, keyboard, printer, display, andthe like. The processing unit 901 may include a central processing unit(CPU) 910, memory 920, a mass storage device 930, a network interface950, and an I/O interface 960 connected to a bus 940.

The bus 940 may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. The CPU 910 may comprise any type of electronic dataprocessor. The memory 920 may comprise any type of system memory such asstatic random access memory (SRAM), dynamic random access memory (DRAM),synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof,or the like. In an embodiment, the memory 920 may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms.

The mass storage device 930 may comprise any type of storage deviceconfigured to store data, programs, and other information and to makethe data, programs, and other information accessible via the bus 940.The mass storage device 930 may comprise, for example, one or more of asolid state drive, hard disk drive, a magnetic disk drive, an opticaldisk drive, or the like.

The I/O interface 960 may provide interfaces to couple external inputand output devices to the processing unit 901. The I/O interface 960 mayinclude a video adapter. Examples of input and output devices mayinclude a display coupled to the video adapter and amouse/keyboard/printer coupled to the I/O interface. Other devices maybe coupled to the processing unit 901 and additional or fewer interfacecards may be utilized. For example, a serial interface such as UniversalSerial Bus (USB) (not shown) may be used to provide an interface for aprinter.

The processing unit 901 may also include one or more network interfaces950, which may comprise wired links, such as an Ethernet cable or thelike, and/or wireless links to access nodes or different networks. Thenetwork interface 901 allows the processing unit to communicate withremote units via the networks 980. For example, the network interface950 may provide wireless communication via one or moretransmitters/transmit antennas and one or more receivers/receiveantennas. In an embodiment, the processing unit 901 is coupled to alocal-area network or a wide-area network for data processing andcommunications with remote devices, such as other processing units, theInternet, remote storage facilities, or the like.

Although the description has been described in detail, it should beunderstood that various changes, substitutions and alterations can bemade without departing from the spirit and scope of this disclosure asdefined by the appended claims. Moreover, the scope of the disclosure isnot intended to be limited to the particular embodiments describedherein, as one of ordinary skill in the art will readily appreciate fromthis disclosure that processes, machines, manufacture, compositions ofmatter, means, methods, or steps, presently existing or later to bedeveloped, may perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein. Accordingly, the appended claims are intended to include withintheir scope such processes, machines, manufacture, compositions ofmatter, means, methods, or steps.

What is claimed is:
 1. A method in a controller for downlink channelsounding in a frequency division duplex wireless system, the methodcomprising: determining with a controller a schedule for an uplinksounding window in a downlink frequency band, wherein the uplinksounding window comprises a transmission window in at least a partialdownlink frequency band that is reserved for uplink channel sounding;instructing a transmission point (TP) to signal the schedule to at leastone wireless device in a coverage area of the TP; receiving a channelsounding signal in the downlink frequency band from the at least onewireless device; and obtaining downlink channel state information at theTP from the channel sounding signal.
 2. The method of claim 1, whereinthe determining with the controller the schedule includes determiningthe schedule in accordance with a number of active wireless devices inthe coverage area.
 3. The method of claim 1, wherein the determiningwith the controller the schedule includes determining the schedule inaccordance with a mobility of the at least one active wireless device.4. The method of claim 1, wherein the determining with the controllerthe schedule includes determining the schedule in accordance with atleast one of a transmission mode and a traffic type of at least one ofthe wireless devices.
 5. The method of claim 1, wherein the determiningwith the controller the schedule includes determining the scheduleincludes dynamically determining the schedule.
 6. The method of claim 1,wherein the determining with the controller the schedule includesdetermining the schedule includes semi-statically determining theschedule.
 7. The method of claim 1, further comprising sending uplinksounding window configuration information to a neighbor TP, wherein theuplink sounding window configuration information comprises a timeduration for the uplink sounding window.
 8. The method of claim 7,wherein sending the uplink sounding window configuration information tothe neighbor TP comprises sending the uplink sounding windowconfiguration information via a backhaul network.
 9. The method of claim1, further comprising sending uplink sounding window configurationinformation to a neighbor TP, wherein the uplink sounding windowconfiguration information comprises information of which UEs to bescheduled.
 10. The method of claim 1, further comprising receivingneighbor uplink sounding window configuration information from aplurality of neighbor TPs, wherein the neighbor uplink sounding windowconfiguration information comprises a time duration for the uplinksounding window and the UEs to be scheduled by a corresponding neighborTP.
 11. The method of claim 10, further comprising sending the neighboruplink sounding window configuration information to the neighbor TPs.12. The method of claim 1, wherein determining with the controller theschedule comprises scheduling the uplink sounding window at a differenttime or frequency from a time or frequency when a neighbor TP isscheduled to transmit a downlink signal to a neighbor wireless device inan edge of a coverage area of the neighbor TP.
 13. The method of claim1, further comprising beamforming a downlink signal to one of thewireless devices to enhance a downlink signal strength to the one of thewireless devices.
 14. The method of claim 1, wherein at least one of thewireless devices comprises multiple receiver antennas to rejectinterference caused by a neighbor wireless device performing channelsounding on the downlink frequency band when the at least one of thewireless devices is receiving a downlink signal from the TP.
 15. Themethod of claim 1, wherein the controller comprises the TP.
 16. Anetwork component configured for downlink channel sounding in afrequency division duplex wireless system comprising: a processor; and acomputer readable storage medium storing programming for execution bythe processor, the programming including instructions to: determine aschedule for an uplink sounding window in a downlink frequency band,wherein the uplink sounding window comprises a time window in at least apartial downlink frequency band that is reserved for uplink channelsounding; instruct a transmission point (TP) to signal the schedule toat least one wireless device in a coverage area of the TP; receive achannel sounding signal in the downlink frequency band from the at leastone wireless device; and obtain downlink channel state information atthe TP from the channel sounding signal.
 17. The network component ofclaim 16, wherein the instructions to determine the schedule includeinstructions to determine the schedule in accordance with a number ofactive wireless devices in the coverage area.
 18. The network componentof claim 16, wherein the instructions to determine the schedule includeinstructions to determine the schedule in accordance with a mobility ofthe at least one active wireless device.
 19. The network component ofclaim 16, wherein the instructions to determine the schedule includeinstructions to determine the schedule in accordance with at least oneof a transmission mode and a traffic type of at least one of thewireless devices.
 20. The network component of claim 16, wherein theinstructions to determine the schedule include instructions todynamically determine the schedule.
 21. The network component of claim16, wherein the programming further comprises instructions to senduplink sounding window configuration information to a neighbor TP,wherein the uplink sounding window configuration information comprises atime duration for the uplink sounding window.
 22. The network componentof claim 21, wherein the instructions to send the uplink sounding windowconfiguration information to the neighbor TP comprises instructions tosend the uplink sounding window configuration information via a backhaulnetwork.
 23. The network component of claim 16, wherein the programmingfurther comprises instructions to send uplink sounding windowconfiguration information to a neighbor TP, wherein the uplink soundingwindow configuration information comprises information of which UEs tobe scheduled.
 24. The network component of claim 16, wherein theprogramming further comprises instructions to receive neighbor uplinksounding window configuration information from a plurality of neighborTP, wherein the neighbor uplink sounding window configurationinformation comprises a time duration for the uplink sounding window andthe UEs to be scheduled by a corresponding neighbor TP.
 25. The networkcomponent of claim 24, wherein the programming further comprisesinstructions to send the neighbor uplink sounding window configurationinformation to the neighbor TPs.
 26. The network component of claim 16,wherein the instructions to determine the schedule comprise instructionsto schedule the uplink sounding window at a different time or frequencyfrom a time or frequency when a neighbor TP is scheduled to transmit adownlink signal to a neighbor wireless device in an edge of a coveragearea of the neighbor TP.
 27. The network component of claim 16, whereinthe programming further comprises instructions to perform beamforming ona downlink signal to one of the wireless devices to enhance a downlinksignal strength to the one of the wireless devices.
 28. The networkcomponent of claim 16, wherein at least one of the wireless devicescomprises multiple receiver antennas to reject interference caused by aneighbor wireless device channel sounding on the downlink frequency bandwhen the at least one of the wireless devices is receiving a downlinksignal from the TP.
 29. The network component of claim 16, wherein thenetwork component comprises the TP.
 30. A method in a network componentfor determining downlink channel state information in a frequencydivision duplex wireless system, the method comprising: determining withthe network component a schedule for transmissions, wherein the schedulecomprises a downlink (DL) transmission period for DL data transmissionin a DL frequency carrier to wireless devices in a coverage area and anuplink (UL) sounding period for UL channel sounding in the DL frequencycarrier; signaling the schedule to the wireless devices; transmitting DLdata to at least one of the wireless devices during the DL transmissionperiod; receiving UL sounding signals in the DL frequency carrier fromat least one of the wireless devices during the UL sounding period; anddetermining DL channel state information according to the UL soundingsignals.
 31. The method of claim 30, further comprising beamforming a DLsignal to one of the wireless devices to mitigate interference fromanother wireless device.
 32. The method of claim 30, further comprisesdetermining a schedule for DL transmission from a neighbor transmissionpoint (TP) and determining the UL sounding period such that the ULsounding period is scheduled for a different time or frequency than theDL transmission from the neighbor TP.
 33. The method of claim 30,wherein determining the UL sounding period comprises determining anumber of active wireless devices in the coverage area.
 34. The methodof claim 30, wherein a duration of the UL sounding period is differentthan a duration of a subsequent UL sounding period.